If you're referring to demonic pacts, well that particular kind of phenomena is quite a rarity I would think, so I doubt you'll meet much of anyone with direct experience.
But I'll have to disagree with the general consensus already here and say that they most certainly are a thing, and quite like you'd imagine.
Usually how it goes, is some sorcerer or power-hungry occultist manages to evoke such a demon, and, depending on the maturity and development of that person's soul and spirit, the demon may decide to entertain their whims. The more potent and mature the sorcerer is, the more useful they would be and the more likely the demon will offer a pact to possess their soul/spirit after death in return for the power or wishes they seek. Also though, the demon will take into account the sorcerer's karmic details, so it won't necessarily fulfill their requests if what the sorcerer is asking for would be too disruptive or heavy on that sorcerer's karmic fate - demons may be negative entities, but they take heed of Providence too and only do as they're allowed like any other spirit.
Anyways, literally speaking, the process is like this:
After the evocation and the conclusion is made by both parties to carry out a pact, they discuss all the terms necessary for it - the services and wishes the demon will fulfill, the capabilities given to the sorcerer, the duties of both parties from then on, the matter of potential servants(familiar spirits) given to the sorcerer by the demon and how they are to be treated, and etc.
Of particular note though, is the negotiation of the sorcerer's date of death, and the means through which he will die and be transferred to the demon's sphere after death.
The whole pact is written down by the sorcerer himself, in which usually two copies are made - one for the sorcerer to keep, and one for the demon. Once it is settled, it is signed by the sorcerer, and then the demon signs it himself with his own seal by using the sorcerer's hand as a medium.
And of course the sorcerer keeps the one copy, while the other is burned, releasing it to the relevant demon's sphere in the process.
Afterwards, the demon will spend the next several months teaching the sorcerer how to use the powers and capabilities given to him, and from there they are free to carry on as settled in the pact. And of course, once the date of death comes, the sorcerer dies as was negotiated and is transferred to the demon's sphere in order to fulfill the pact and pay his debts for the demon's services.
It's true that spirits like veneration and having some helping hands on earth, but not only are most potent, worthwhile spirits capable of influencing the physical world on their own, there are also plenty of reasons for them to possess a human soul/ spirit as a servant as well - and demons, particularly, have no qualms taking advantage of that unique usefulness.
Humans are tetrapolar spirits made in the image of God, which gives them many more capabilities among all the spheres than what a demon who would make such a pact would have. For example, it us much easier for human spirits to travel amongst all the different spheres, while demons(and most other spirits) are mostly dependent on residing in their own zone. This makes humans good messengers and servants for the demon outside of their own sphere, and it is a common way the souls/spirits of humans are utilized in the case of pacts.
Even more commonly, however, are demons assigning the indentured souls/spirit of these humans as servants for other sorcerers still involved with them on the earth sphere - so literally, you can be someone who makes a pact, die and go to serve the demon, and then end up back on earth as someone else's familiar spirit who has made their own deal with said demon.
Anyways, making pacts like this really aren't a good thing in the long run and should be avoided, obviously
Not only is it probably not worth the servitude afterwards, but if you're stuck serving a demon for long enough, you'll start to de-evolve by the influence of them and their sphere's energies, eventually making your soul/spirit dependent on them. Meaning, that you'll eventually transmute into a demon yourself, and be bound to their sphere forever, like they are.
Actually you're right, but not only is it referenced in his Frabato autobiography, it is directly elaborated upon in his "The Practice of Magical Evocation" book as well.
